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Effects of Purine Nucleotides on the Binding of [ 3 H]Cyclopentyladenosine to Adenosine A‐l Receptors in Rat Brain Membranes
Author(s) -
Williams Michael,
Braunwalder Albert
Publication year - 1986
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1986.tb02835.x
Subject(s) - adenosine , gtp' , adenosine deaminase , chemistry , purinergic receptor , nucleotide , adenosine triphosphate , adenosine a2b receptor , biochemistry , adenylate kinase , receptor , adenosine receptor , enzyme , agonist , gene
Adenine nucleotides displace the binding of the selective adenosine A‐l receptor ligand [ 3 H]cyclopentyl‐adenosine (CPA) to rat brain membranes in a concentration‐dependent manner, with the rank order of activity being ATP > ADP > AMP. Binding was also displaced by GTP, ITP, adenylylimidodiphosphate (AppNHp), 2‐meth‐ylthioATP, and the β‐γ‐methylene isostere of ATP, but was unaffected by the α‐β‐methylene isosteres of ADP and ATP, and UTP. At ATP concentrations >100 μ M , the inhibitory effects on CPA binding were reversed, until at 2 m M ATP, specific binding of CPA was identical to that seen in controls. Concentrations of ATP >10 m M totally inhibited specific binding. Inclusion of the catabolic enzyme adenosine deaminase in the incubation medium abolished the inhibitory effects of ATP, indicating that these were due to adenosine formation, presumably due to ectonucleotidase activity. The inhibitory effects were also attenuated by the α‐β‐methylene isostere of ATP, an ectonucleotidase inhibitor. Adenosine deaminase, α‐β‐methylene ATP (100 μ M ), and β‐γ‐methylene ATP (100 μ M ) had no effect on the “stimulatory” phase of binding, although GTP (100 μ M ) slightly attenuated it. Comparison of the binding of [ 3 H]CPA in the absence and presence of 2 m M ATP by saturation analysis showed that the K D and apparent B max values were identical. Examination of the pharmacology of the control and “ATP‐dependent” CPA binding sites showed slight changes in binding of adenosine agonists and antagonists. The responses observed with high concentrations of ATP were not observed with GTP, AppNHp, the chelating agents EDTA and EGTA, or inorganic phosphate. The divalent cations Mg 2+ and Ca 2+ at 10 m M attenuated the stimulatory actions of high (2 m M ) concentrations of ATP, whereas EGTA and EDTA (10 m M ) enhanced the “stimulatory” actions of ATP. EDTA (10 m M ) abolished the inhibitory effects of ATP, indicating a specific dependence on Mg 2+ for the inhibitory response. The effects of ATP on [ 3 H]CPA binding were reversible for antagonists but not agonists. The mechanism by which ATP reverses its own inhibitory action on adenosine A‐l radioligand binding is unclear, and from the observed actions of the divalent cations and chelating agents probably does not involve a phosphorylation‐dependent process.